The present invention relates to a circuit pattern inspecting instrument and a circuit pattern inspecting method, and in particular to an instrument and a method for inspecting circuit patterns on a wafer or the like in a semiconductor device manufacturing process.
For a comparison testing method of detecting defects in circuit patterns formed on a wafer in a semiconductor device manufacturing process, there has been put to practical use an instrument for inspecting the wafers by comparing images of two or more LSI circuits of the same pattern formed on a single wafer with each other.
Particularly, instruments for pattern comparison and pattern inspection using an electron beam are described in Japanese Patent Application Laid Open No. Sho 59-192943; P. Sandland et al., xe2x80x9cAn electron-beam inspection system for x-ray mask productionxe2x80x9d, Journal of Vacuum Science and Technology B, Vol. 9, No. 6, November/December 1991, pp. 3005-3009; D. Fleming et al., xe2x80x9cProspects for x-ray lithographyxe2x80x9d, Journal of Vacuum Science and Technology B, Vol. 10, No. 6, November/December 1992, pp. 2511-2515; D. Hendricks et al., xe2x80x9cCharacterzation of a New Automated Electron-Beam Wafer Inspection System, Integrated Circuit Metrology, Inspection, and Process Control IX, February 20-22, 1995, Santa Clara, CA, Proceedings of SPIE, Vol. 2439, May 1995, pp. 174-183; and Japanese Patent Application Laid Open No. Sho 5-258703. In those instruments, for obtaining a practical throughput, it is necessary to acquire images at a very high speed, and at least 100 times (at least 10 nA) the electron beam current with an ordinary scanning electron microscope is used to ensure a sufficient S/N ratio of the images acquired at high speed as well as a practical inspection speed. The electron beam diameter is spread fairly wider than that in an ordinary scanning electron microscope and is about 0.05 to 0.2 xcexcm. This is because of an increase in chromatic aberration caused by widening of the electron energy width which is attributable to a large beam current, a limitation to brightness of an electron gun and a limitation by the Coulomb effect.
The images formed by such an electron optic system are fed to an image processing unit, in which images of the adjacent circuits of the same pattern are compared with each other for inspection. If a portion having different brightness occurs between the compared images, the portion is regarded as a defect and coordinates of the portion are stored.
With the above configuration, it is possible to detect even a defect as small as 0.1 xcexcm or so.
Further, instruments for inspecting defects in semiconductors by reducing the energy of the electron beam with a voltage applied to a sample and an electrode disposed close to the sample are disclosed in Japanese Patent Application Laid Open Nos. Hei 7-78855, 9-181139, 10-19538, 10-27834, 10-27835, and 11-25901. But these references do not describe an instrument having a review function to be described later, or an instrument having a combination of the review function and an energy analyzing function to be described later.
Before an inspection is started using one of the above instruments, there are various parameters to be set in advance. As parameters to be set for an electron optic system there are an irradiation energy of an electron beam, a gain of a signal detection system for image forming. secondary electrons (or charged particles such as back-scattered electrons), a pixel size (a minimum picture unit), and the amount of a beam current. On the other hand, it is necessary to set a threshold value for judging whether a signal indicates a defect in comparison of two images obtained from the two adjacent areas of the same pattern by an image processing unit. If this threshold value is set too low, the defect detection sensitivity is high, but it increases a possibility that a faultless portion is judged defective. On the other hand, if the threshold value is set too high, the detection sensitivity becomes too low.
Optimum values of the above parameters differ depending on a process to be inspected, a pattern size, and a type of defects to be inspected. Therefore, it is necessary to optimize the parameters by conducting a test inspection in which an image at the coordinates of a detected defect is displayed to confirm that a defect desired to be detected has been detected, before a regular inspection.
It is also necessary to for an operator to obtain the image at the coordinates of the defect after the inspection and check what kind of defects has been detected.
In addition to acquiring an image at a high speed so as to see whether a defect is present and then detecting a defect by processing the image, it is also essential to produce an image of a specific small area and observe the image visually as in the case with an ordinary scanning electron microscope.
A mode for this observation will be hereinafter referred to as xe2x80x9ca review mode.xe2x80x9d in this specification.
If it is necessary to make distinction between this review and the inspection mode based on high-speed acquisition of images for detecting the presence of a defect over a relatively large area, the inspection mode based on high-speed acquisition of images will be referred to as xe2x80x9ca defect detecting inspection.xe2x80x9d
For the review, it is not necessary to form images at such a high speed as in the defect detecting inspection, but a high resolution image is needed because it is necessary, not only to recognize whether a defect is present or not, but also to recognize the shape and type of the defect to some extent.
In the conventional instruments, however, an electron optic system used therein is designed so as to be best suited for the acquisition of an image by high-speed scanning at a large current, and it has so far been impossible to obtain a resolution high enough for images for the review. Consequently, it was impossible to judge accurately whether a detected defect is a true defect or a false defect due to an erroneous detection caused by inappropriate setting of parameters. Accordingly, inspection has often been conducted with the parameters being not set to optimum values.
It is an object of the present invention to provide an inspecting instrument making possible efficient setting of the conditions for inspecting with an electron beam, defects in repeating design patterns, foreign matters, residues and the like in a semiconductor device on a wafer in a semiconductor device manufacturing process, for example.
According to the present invention, the above-mentioned object is achieved by the following configurations.
According to an aspect of the present invention, a circuit pattern inspecting instrument includes a cathode for emitting an electron beam; a stage for mounting a sample thereon; an electron-optical system for focusing the electron beam; a deflector for scanning the electron beam on the sample; a detector for detecting secondary charged particles from the sample irradiated by the electron beam; and a mode setting unit for switching between a first mode and a second mode; wherein in the first mode, a current of the electron beam is set to a first value and the electron beam is scanned at a first speed; wherein in the second mode, the current of the electron beam is set to a second value smaller than the first value and the electron beam is scanned at a second speed lower than the first speed; and wherein the circuit pattern inspecting instrument is configured so that first the sample is observed in the first mode, then a particular position on the sample is selected based on image data produced by an output of the detector in the first mode, and then the particular position on the sample is observed in the second mode.
According to another aspect of the present invention, a circuit pattern inspecting instrument includes a first electron-optical system including a first cathode for emitting a first electron beam, a first objective lens having a first focal length for focusing the first electron beam on a sample positioned at a first sample position, and a first scanning deflector for scanning the first electron beam on the sample positioned at the first sample position; a first detector for detecting secondary charged particles generated from the sample positioned at the first sample position; a second electron-optical system including a second cathode for emitting a second electron beam, a second objective lens having a second focal length shorter than the first focal length for focusing the second electron beam on a sample positioned at a second sample position, and a second scanning deflector for scanning the second electron beam on the sample positioned at the second sample position; and a second detector for detecting secondary charged particles generated from the sample positioned at the second sample position; an image-forming device for imaging the sample positioned at the first sample position based on an output of the first detector, and for imaging the sample positioned at the second sample position based on an output of the second detector; and a stage for moving a sample between the first sample position and the second sample position; wherein the first electron-optical system, the second electron-optical system, the first detector, the second detector, and the stage are housed in a single vacuum chamber; and wherein the circuit pattern inspecting instrument is configured so that first the sample is observed at the first sample position with a current of the first electron beam being set to a first value and the first electron beam being scanned at a first speed, then a particular position on the sample is selected based on image data produced by an output of the first detector, then the particular position on the sample is moved to the second sample position by moving the stage, and then the particular position on the sample is observed by enlarging the particular position on the sample using the second electron-optical system with a current of the second electron beam being set to a second value smaller than the first value and the second electron beam being scanned at a second speed slower than the first speed.
According to another aspect of the present invention, there is provided a method of inspecting a circuit pattern includes the steps of (a) detecting, using a frist detector disposed at a first position, secondary charged particles from a sample mounted on a stage and irradiated by a first electron beam scanning the sample at a first scanning speed with a current of the first electron beam being set to a first value; and (b) detecting, using a second detector disposed at a second position different from the first position, secondary charged particles from a particular position on the sample irradiated by a second electron beam scanning the sample at a second scanning speed lower than the first scanning speed with a current of the second electron beam being set to a second value lower than the first value, the particular position being selected based on an output of the first detector.
According to another aspect of the present invention, a method of inspecting a circuit pattern includes the steps of (a) providing an electron-optical system for irradiating and scanning a sample having a circuit pattern thereon by a focused electron beam, a detector for detecting back-scattered electrons or secondary electrons from an electron beam-irradiated portion of the sample, an image forming unit for forming an image of the sample based on a detected signal from the detector, and a difference detecting circuit for comparing an image signal obtained by the image forming unit with a reference image signal and thereby detecting a difference between the two image signals; (b) amplifying an output from the detector using an amplifier having a first amplification factor, the output being obtained by, scanning a relatively large region of the sample with the electron beam of a relatively large electric current at a relatively high-speed; (c) then supplying the thus-amplified output to the image forming unit to form an image signal; (d) comparing the image signal with a similar image signal obtained from another region of the sample, so as to detect a difference between the image signals; (e) determining coordinates of a position where the difference has occurred; (f) scanning a region of a smaller area than the relatively large region, including the position of occurence of the difference, with the electron beam of a smaller electric current than the relatively large electric current and at a lower speed than the relatively high speed; (g) then supplying the resulting output from the detector to the image forming unit via a circuit provided with an amplifier which amplifies the output at a larger amplification factor than the first amplification factor and is provided with a high-frequency component cut-off filter, to form an image signal; and (h) observing the difference-generating position.